Delaware Route 404 (DE 404) is a major state highway in Sussex County, Delaware that spans the width of the state. DE 404's western terminus is at the Maryland border northwest of Bridgeville, where it continues as Maryland Route 404 (MD 404), and its eastern terminus is at the Five Points intersection with U.S. Route 9 (US 9), DE 1, and US 9 Business (US 9 Bus.). The route passes through rural areas as well as the towns of Bridgeville and Georgetown. DE 404 runs concurrent with DE 18 from east of Bridgeville to Georgetown and with US 9 from Georgetown to Five Points. DE 404 has a business route, DE 404 Bus., that passes through Bridgeville. DE 404, along with MD 404, serves as a major route connecting the Chesapeake Bay Bridge and the Baltimore-Washington Metropolitan Area with the Delaware beaches.

DE 404 was first designated by 1936 to run from the Maryland border to DE 18 southeast of Bridgeville. The route was extended to Five Points by 1987. By 1999, DE 404 was aligned to bypass Bridgeville, with DE 404 Bus. designated on the former route through the town. The new alignment of DE 404 around Bridgeville replaced DE 404 Alternate (DE 404 Alt.)

DE 404 begins at the Maryland border, where it continues northwest into that state as MD 404. From the state line, the route heads southeast on two-lane undivided Seashore Highway, passing through a mix of farmland and woodland with some homes. In the community of Scotts Corner, the road intersects the western terminus of DE 36. DE 404 continues through more rural areas and turns northeast onto Newton Road, with DE 404 Bus. continuing southeast along Seashore Highway toward Bridgeville. From here, DE 404 heads through agricultural areas to the north of Bridgeville, curving east and crossing Norfolk Southern's Delmarva Secondary railroad line. The route comes to an intersection with US 13 and turns south to form a concurrency with that route on Sussex Highway, a four-lane divided highway. A short distance later, US 13 Bus. splits off to the southwest to head into Bridgeville. The road continues through farmland with some businesses to the east of Bridgeville. Upon reaching an intersection with US 13 Bus./DE 404 Bus., DE 404 splits from US 13 by heading southeast onto four-lane divided Seashore Highway. The road passes a few businesses before becoming two lanes and undivided as it continues through agricultural areas, coming to an intersection with DE 18.

At this point, DE 18 heads east concurrent with DE 404 along Seashore Highway, running through a mix of farmland and woodland with some residences. The road heads through more forested areas with some agriculture and homes, passing through tracts of the Redden State Forest. Farther east, DE 18/DE 404 heads into Georgetown, passing to the north of Delaware Technical Community College's Jack F. Owens Campus. The road crosses US 113 in a commercial area and continues east. The two routes head southeast onto North Bedford Street, passing homes and businesses. The road comes to an intersection with US 9 at The Circle in the center of Georgetown, where DE 18 ends.

At this point, DE 404 continues northeast concurrent with US 9 on East Market Street, lined with downtown businesses. The road crosses Norfolk Southern's Indian River Secondary railroad line and runs through residential areas with a few commercial establishments. US 9/DE 404 heads east-northeast out of Georgetown and becomes Lewes Georgetown Highway, passing through farmland with some homes and businesses. The two routes intersect the eastern terminus of US 9 Truck and run through wooded areas with some fields and development. In Gravel Hill, the road intersects DE 30. Following this intersection, US 9/DE 404 passes through more farmland and woodland with some residences, reaching a junction with DE 5 in Harbeson. The two routes run through more rural areas, crossing a Delaware Coast Line Railroad line. The road heads through farmland with increasing residential subdivisions. Upon reaching the area of the Five Points intersection in Nassau, US 9/DE 404 runs past homes and businesses, widening into a divided highway and coming to an intersection with DE 1D/DE 23. DE 23 joins US 9/DE 404 for a concurrency on a four-lane divided highway before the road comes to a junction with DE 1. Here, DE 23/DE 404 ends, US 9 Bus. continues northeast, and US 9 heads east for a concurrency with DE 1.

DE 404 serves as a major route connecting the Chesapeake Bay Bridge and the Baltimore-Washington Metropolitan Area to the Delaware beaches. The highest annual average daily traffic along DE 404 is 30,773 vehicles along the US 13 concurrency with the lowest being 5,777 vehicles at the western terminus of DE 404 Business. All of DE 404 is part of the National Highway System with the exception of the portion along Newton Road in Bridgeville.

By 1920, what would originally become DE 404 was an unimproved county road, with the section east of Bridgeville under contract as a state highway. The entire route of what would become DE 404 was completed as a state highway to a point northwest of Bridgeville by 1924. By 1931, the road was built as a state highway northwest to the Maryland border. When Delaware designated its state highways by 1936, DE 404 was designated to run from the Maryland border, where it connected to MD 404, southeast to DE 18 southeast of Bridgeville. The route followed its current alignment to Bridgeville, where it passed through the town on Market Street and South Main Street before picking up its current alignment east of US 13. At this time, what is now DE 404 past its then-eastern terminus was designated as part of DE 18. By 1987, DE 404 was extended east to DE 1 in Five Points, following DE 18 to Georgetown and US 9 (which had replaced a portion of DE 18) from Georgetown to Five Points. By 1990, DE 404 Alt. was created to bypass Bridgeville to the north and east, following Newton Road and US 13. DE 404 Alt. became a part of DE 404 by 1999, with the former DE 404 through Bridgeville becoming DE 404 Bus.

The entire route is in Sussex County.

Delaware Route 404 Business (DE 404 Bus.) is a 3.34-mile (5.38 km) long business route of DE 404 that runs through the town of Bridgeville. The route begins at DE 404 northwest of Bridgeville, heading southeast on two-lane undivided Seashore Highway through agricultural areas with some homes. DE 404 Bus. enters Bridgeville and heads through commercial areas, turning northeast onto Market Street and crossing Norfolk Southern's Delmarva Secondary railroad line. The route passes several homes before intersecting US 13 Bus. and turning south to join that route on South Main Street. The two routes run through more residential areas before leaving Bridgeville and heading through farmland with some development. The road widens into a four-lane divided highway and heads into a commercial area, intersecting US 13 and DE 404. At this point US 13 Bus. and DE 404 Bus. both end, with the road continuing southeast as DE 404. DE 404 Bus. was created in 1999 when DE 404 was rerouted to bypass Bridgeville on the alignment of DE 404 Alt.

Delaware Route 404 Alternate (DE 404 Alt.) was a 4.65-mile (7.48 km) alternate alignment of DE 404 that bypassed the town of Bridgeville. It ran to the north of Bridgeville on Newton Road and to the east of Bridgeville on US 13. The route was created by 1990. The alignment of DE 404 Alt.was replaced by DE 404 itself in 1999. The route of DE 404 in Bridgeville then became known as DE 404 Bus.